Betamon X: ACK! All this debate we've missed out on.
Gomamon: NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!
Alright, I've seen so much I just love in this thing, debate, facts, tearing down facts, tearing down the tearing down of facts, obscure little boosters.
Now... It's my turn. First things first, as far as "smart" goes, Tohma obviously knows a whole heck of a lot more then izzy. For evidence, go back and look at everything that has been said.
However... Izzy is the smarter one.
A. What is the definition of "Smart?" If it's how much stuff a person can remember (aka... I.Q.) then it's a battle between Henry and Joe. However, Izzy may not be able to find the same kind of facts and understand everything like Touma, he does know where to get it. That makes him smarter, because he has a more available access to all knowledge then Tohma, and can interrpret it extremly easily.
B. Tohma is a realist, Izzy is not. Izzy came up with theories and ideas about that digital world, even though some of them ended up being not true, he wasn't to look at this brand new world and just go cool, Izzy's curiousity drove him, Tohma's fear for his sister's life. Reason is the biggest thing behind a person's degree of "Smartness."
C. Izzy is more like henry, and Henry is the smartest out of all the seasons... which makes me wonder why he wasn't in the available options.